WhatsApp has revealed a vulnerability in its system that could have allowed hackers access to its users’ phones, with a London-based human rights lawyer possibly among the targets.

The encrypted messaging service, owned by Facebook, said Monday that it had discovered and fixed the vulnerability the attackers had sought to exploit. The hackers could implant malicious code on a victim’s phone by placing a voice call to the victim on WhatsApp.

“The attack has all the hallmarks of a private company reportedly that works with governments to deliver spyware that takes over the functions of mobile phone operating systems,” a WhatsApp spokesperson said in a statement.

In a statement provided to CNN on Monday, NSO said, “Under no circumstances would NSO be involved in the operating or identifying of targets of its technology, which is solely operated by intelligence and law enforcement agencies.”

NSO said its technology was licensed to government agencies “for the sole purpose of fighting crime and terror,” adding that those agencies determine how the technology is used without any involvement from the company.

Human rights activists targeted?

Among thosebelieved to have been targeted via WhatsApp is a London-based human rights lawyer.

On Sunday, the lawyer received two calls that John Scott-Railton, a senior researcher at the University of Toronto’s Citizen Lab believes were part of the attack. Citizen Lab is an academic security research group that investigates digital threats to civil society groups and online freedom of expression.

The apparent attempt to breach the lawyer’s phone was not successful, Scott-Railton said, as WhatsApp had patched the vulnerability by Sunday.

WhatsApp had reached out to Citizen Lab and a number of other groups that work with human rights defenders before publicly acknowledging the attack.

The collaboration between WhatsApp and Citizen Lab helped identify the attempted attack on the London-based lawyer. The lawyer does not want to be named, Scott-Railton told CNN.

Responding specifically to the apparent targeting of the lawyer, NSO Group said in a statement, “NSO would not or could not use its technology in its own right to target any person or organization, including this individual.”

The group claims that NSO software “threatens the rights to privacy and to freedom of opinion and expression, in breach of Israel’s obligations under international human rights law.”

It said one of its researchers had been targeted via a WhatsApp message containing NSO’s spying software in 2018 while working on a campaign to release six women’s rights activists detained in Saudi Arabia.

How to update your WhatsApp

WhatsApp said while it has fixed the vulnerability the attackers were exploiting, it is also encouraging users to update to the latest version of the WhatsApp app “out of an abundance of caution.” The company said it has also contacted US law enforcement.

Ireland’s Data Protection Commission, which supervises Facebook’s activities in Europe, said it had been informed of the vulnerability on Monday, adding it was unclear at this stage whether any EU user data had been affected.

Still, it too urged users to ensure the update WhatsApp on their devices.

Here’s how:

On an iPhone

— Open the App Store and select updates.

— Select “WhatsApp” and Update.

On an Android device

— Open the Play Store and tap on the 3 lines in the upper left corner.